1. Field of the Invention
The present invention relates to glass compositions which are uniquely applicable to the preparation of ultrafine fibers for filtration and separation applications. Fiber made from the glasses disclosed exhibit the necessary properties of moisture resistance, chemical resistance, strength, and biosolubility.
2. Description of the Related Art
Glass forming compositions suitable for fiberization are typically restricted by their melt and end properties to conform to process specifications and product performance criteria. For example, in both rotary and flame attenuation processes, only certain values for high temperature viscosity (T at 103 poise) and liquidus are acceptable. Moreover, such compositions must demonstrate adequate physical properties such as tensile strength and moisture resistance when formed into fibers by these processes. In addition, more recently, it has become increasingly more important that these fibers degrade at sufficiently high rates in the body, such that they pose little to no potential risk to humans if inhaled and can at least be demonstrated to have limited biological effects to laboratory animals when tested.
A good glass fiber forming composition should also have good xe2x80x9crunabilityxe2x80x9d, i.e., the ability to be easily fiberized into long fibers of small diameter with good production rates and little or no shot. While there are many factors involved in this, not all of which have been clearly identified, it is believed that surface tension and lack of tendency for the melt phase to separate play key roles. In specific, it is desirable for a glass composition to have as low a surface tension as possible at fiberization temperatures (keeping in mind the other factors above), such that the work done in forming a unit area of surface is kept to a minimum.
All of these factors are especially important in the production of ultrafine fibers used to produce specialty papers and other media for air and liquid filtration applications. Such applications place some significant demands on the glass in terms of both fiberization and end properties.
Among the specific requirements of such glasses are the melting and fiberization temperatures. The glass must be capable of being melted and fiberized at temperatures low enough for the capability of the equipment and to attain reasonable (economic) production rates. This requires that the HTV value of the glass (temperature when the melt viscosity is 1000 poise) be less than 2200xc2x0 F. (1204xc2x0 C.), preferably less than 2000xc2x0 F. (1093xc2x0 C.), and the surface tension of the glass at 1652xc2x0 F. (900xc2x0 C.) be less than 315 dyne/cm.
The glass must not crystallize or devitrify in the melters, pots, bushings or anywhere in the system used to melt, contain, transport, or fiberize the glass. Crystallization impairs flow of melt to fiberization orifices. To prevent devitrification and provide a good working range for fiberization, the liquidus temperature of the glass must be at least 80xc2x0 F. (44xc2x0 C.) and preferably at least 150xc2x0 F. (83xc2x0 C.) below the HTV temperature.
The glass must not corrode or have adverse reactions with metal parts or refractories used to contain the melt or the newly-forming fiber. Also, the glass must be capable of being drawn into ultrafine diameters (as low as 0.2 xcexcm) without breakage into short lengths.
The glass must not produce excessive volatilization during melting or fiberizing. Volatilization leads to compositional variations, increased corrosion of refractories, increased emissions, and, when volatiles condense, to unacceptable dust levels in product. As well, the glass must provide strength to the fiberxe2x80x94suitable to form fibers capable of being wet or dry (air) processed into papers or felts and meet all of the requirements (tensile, elongation) required for the paper product.
Because of its high specific surface area, the glass must also be sufficiently chemically durable, particularly with regard to ambient atmospheric moisture, so that little to no deterioration in fiber strength occurs with time during packaging, shipping, and storage prior to use in papermaking process. The moisture resistance of a glass fiber can be correlated with its thermodynamic tendency to react with water as measured by its free energy of hydration. For nearly all commercial glass fiber, this value is negative, indicating that the fibers will over time react. However, it is the rate at which the reaction occurs that is critical. For fiber to perform adequately and preserve tensile properties, the free energy of hydration should not be less than (more negative than) xe2x88x924.5 kcal/mole. Loss in fiber strength also correlates with increase in fiber surface area as measured by BET methods (using krypton). After exposure of fibers to 122xc2x0 F. (50xc2x0 C.) for 72 hours, the change in surface area should be less than 10%. The glass must also be sufficiently durable and resistant to moisture attack after contact with whitewaters or other media used in the papermaking process, so that little to no deterioration occurs with time after fiber is in paper product.
Because the fiber is of very fine diameter and capable of being respired, it must also degrade in the body at a rate that is sufficiently rapid so as not to induce respiratory diseases, especially chronic diseases such as emphysema or cancer. Measured in vitro dissolution rates for the fiber in simulated physiological saline (kdis) must be greater than 100 ng/cm2 hr.
The glass fibers must also be leachable in paper making media such as acid whitewaters such that hydrolytic bonds can form between leached fiber surfaces when the paper is dried. Such bonds provide strength and structural integrity in the final product. However, too great a leach rate can leave the fiber with a porous surface structure which is too susceptible to moisture attack after the paper is formed. Formation of such bonds can be correlated with bulk silica content of the glass. For good gel bonding the silica content must be at least 58 mole %. The glass fibers must also show good performance in handsheets, both in initial tensile strength and in loss in tensile strength over time. This is evaluated by determining load to failure at a gauge length of 4xe2x80x3 (10.2 cm) of mechanically formed handsheets with a nominal basis weight of 0.02 lb/sq. ft. (9.6 mg/cm2). For handsheets made of fiber with a 0.3 xcexcm geometric mean diameter, initial tensile strengths should be at least 3.4 lbs./in. with no statistically significant loss in tensile strength after aging at 95xc2x0 F. (35xc2x0 C.) and 95% relative humidity for up to 168 hrs.
The glass fibers must also show good performance in doubly folded handsheets, both in initial tensile strength and in loss in tensile strength over time. This is evaluated by determining load to failure at a gauge length of 4xe2x80x3 (10.2 cm) for the handsheets whose properties are defined above. For handsheets made of fiber with a 0.3 xcexcm mean diameter, initial folded tensile strengths should be at least 1.8 lbs./in. Tensile strength after aging at 95xc2x0 F. (35xc2x0 C.) and 95% relative humidity for up to 168 hrs should show an exponential decay with a t xc2xd of no less than 250 days. The glass fibers must also show good performance in mat elongation, both as measured in direct and folded tensile tests as described above. Elongation measures the integrity of the fiber to fiber bond and can be related to both manufacturability of the paper and its performance in product (e.g., pleating, etc.). For handsheets made of fiber with a 1 xcexcm mean diameter and basis weight as defined above, elongation in either test should be less than 2% at failure. Change in elongation upon aging under the conditions above should be less than 30%.
The achievement of producing ultrafine glass fibers with the requisite strength, chemical and moisture resistance, while also exhibiting acceptable biosolubility is quite challenging. The industry would find such fiberglass quite useful. Accordingly, glass compositions suitable for efficiently making ultrafine fibers for use in specialty papers and particularly in media for air and liquid filtration applications, where the requested biosolubility and strength performance are realized, are greatly desired.
The present invention defines glass compositions which are uniquely applicable to the preparation of ultrafine fibers for filtration and separation applications. The market for these products is presently served by glass compositions that perform well and meet all of the criteria defined above with the exception of dissolution rate in simulated physiological saline. The glasses of the present invention meet all of these criteria including that for dissolution rate. The compositional range in which glass fibers meeting these criteria can be formed is as follows in mole %:
The glass compositions of the present invention are particularly adapted to provide glass fiber media which is useful for fabrication of HEPA air filtration systems for clean-room installations. Glass HEPA media are made from a blend of fine diameter glass fibers ranging from 0.2 to 2 micron. These products are rated for high filtration efficiency values while maintaining reasonably low pressure-drop characteristics.
The glass composition and fibers of the present invention exhibit stability in wet acidic environments and resistance to moisture attack during storage under humid ambient conditions, which are two important properties of glass fibers used in HEPA filtration applications. In addition, the glasses of the present invention have viscosity and liquidus temperature properties that are compatible with fiberglass manufacturing technologies used to produce fine diameter fibers, particularly flame attenuation processes. The glass compositions and fibers of the present invention also exhibit excellent biosolubility such that the fibers degrade at a high rate in the body if inhaled, which is a property that is becoming increasingly more important.
Among other factors, it has been found that the glass compositions of the present invention can be used to prepare glass fibers, by any method, including flame attenuation or rotary, which exhibit the necessary chemical and strength properties, as well as processing characteristics, to allow for the efficient preparation of the fibers for use in paper and filtration products. Particular application is found for ultrafine fibers in filter products. Moreover, the glass compositions permit such chemical and strength characteristics while also offering good biosolubility. It is through a balance of the components of the glass composition that such a balance of properties is possible, which balance is reflected in the compositions of the present invention.